用C#写一段关于CAD二次开发中多段线偏移的程序，要求线上的点到偏移后的线段离距都是固定的

以下是一个简单的C#程序，用于CAD二次开发中对多段线进行偏移，使得线上的点到偏移后的线段离距固定。该程序使用了CAD的COM接口，需要在程序中引用CAD的类型库。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Geometry;

namespace MyCommands
{
    public class MyCommandsClass
    {
        [CommandMethod("MyOffset")]
        public void MyOffset()
        {
            // 获取当前文档和数据库
            Document doc = Application.DocumentManager.MdiActiveDocument;
            Database db = doc.Database;
            // 提示用户选择多段线
            PromptEntityOptions peo = new PromptEntityOptions("请选择多段线：");
            peo.SetRejectMessage("请选择一条多段线！");
            peo.AddAllowedClass(typeof(Polyline), true);
            PromptEntityResult per = doc.Editor.GetEntity(peo);
            if (per.Status != PromptStatus.OK) return;
            // 打开事务
            using (Transaction trans = db.TransactionManager.StartTransaction())
            {
                // 获取多段线对象
                Polyline pline = trans.GetObject(per.ObjectId, OpenMode.ForRead) as Polyline;
                if (pline == null) return;
                // 提示用户输入偏移距离
                PromptDoubleOptions pdo = new PromptDoubleOptions("请输入偏移距离：");
                pdo.AllowNegative = true;
                PromptDoubleResult pdr = doc.Editor.GetDouble(pdo);
                if (pdr.Status != PromptStatus.OK) return;
                double distance = pdr.Value;
                // 计算多段线的偏移路径
                Polyline offsetPline = new Polyline();
                for (int i = 0; i < pline.NumberOfVertices; i++)
                {
                    Point2d pt = pline.GetPoint2dAt(i);
                    double bulge = pline.GetBulgeAt(i);
                    if (i == 0 || i == pline.NumberOfVertices - 1)
                    {
                        offsetPline.AddVertexAt(i, pt + new Vector2d(distance, 0), bulge, 0, 0);
                    }
                    else
                    {
                        Vector2d vec1 = pt - pline.GetPoint2dAt(i - 1);
                        Vector2d vec2 = pline.GetPoint2dAt(i + 1) - pt;
                        double angle = vec1.GetAngleTo(vec2) / 2.0;
                        double offsetDist = distance / Math.Cos(angle);
                        Vector2d offsetVec = vec1.GetNormal() * offsetDist;
                        offsetPline.AddVertexAt(i, pt + offsetVec, bulge, 0, 0);
                    }
                }
                // 将偏移后的多段线添加到数据库中
                BlockTableRecord btr = trans.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) as BlockTableRecord;
                offsetPline.Layer = pline.Layer;
                btr.AppendEntity(offsetPline);
                trans.AddNewlyCreatedDBObject(offsetPline, true);
                // 提交事务
                trans.Commit();
            }
        }
    }
}

该程序首先提示用户选择一条多段线，然后提示用户输入偏移距离。接着，程序计算多段线的偏移路径，将偏移后的多段线添加到数据库中，并提交事务。注意，在计算偏移路径时，需要考虑多段线的拐弯处，以保证线上的点到偏移后的线段离距固定。